Added read-only option.

I had one major issue with Gollum the way it is currently. It would not allow one to disable editing of the wiki via the web interface. I had a problem with this as it allows anyone on the web to go and miss up my wiki without an approval process or identification as to who they were.

My plan is to put my wiki on github or some other git repository system and allow people to add to the wiki via pull requests and patches. More control as to who can do what and also does not allow any stranger to just go and spam the wiki.

Hope you incorporate my changes in Gollum. I know other people are wanting this.